A Twine is not intended for use directly and should not be stored, its
implementation relies on the ability to store pointers to temporary
stack objects which may be deallocated at the end of a statement.
Twines should only be used accepted as const references in arguments,
when an API wishes to accept possibly-concatenated strings.

In the other words, Twine object doesn't own its parts, so they are destroyed at the end of statement.